Image forming apparatuses read images from documents, form developer images corresponding to the images read, on paper sheets, and fix the developer images on the paper sheets by using a fixing apparatus.
In the fixing apparatus, a paper sheet is held between the heat roller and the press roller, and heat and pressure are applied to the paper sheet. The developer image on the paper sheet is thereby fixed.
A center coil and side coils are provided within or outside the heat roller. These coils generate high-frequency magnetic fields when a high-frequency current is supplied to them. From the high-frequency magnetic fields there are generated eddy currents. The eddy currents turn into Joule heat. The Joule heat heats the heat roller. A fixing apparatus of induction heating type is disclosed in, for example, Jpn. Pat. Appln. KOKAI Publication No. 2001-312178.
The center coil performs induction heating at that part of the heat roller which is almost middle in the axial direction of the heat roller (i.e., the direction at right angles to the direction in which the heat roller rotates). The side coils perform induction heating at one end of the heat roller and the other end thereof, respectively. The center coil and the side coils are alternately driven, each for a prescribed time.
As a paper sheet passes, while nipped between the heat roller and the press roller, however, the heat roller is more heated at the end parts than at the middle part. Thus, the end parts of the press roller rise to a higher temperature than the middle part of the press roller.
The press roller is made of rubber. When its end parts rise to a higher temperature than its middle part, the press roller becomes soft, more quickly at the end parts than at the middle part.
Once the hardness of the end parts of the press roller has more decreased than the middle part of the press roller, the end parts of the press roller cannot transport paper sheets so efficiently as the middle part. Consequently, a paper sheet will be creased.
Further, when the end parts of the press roller rise to a higher temperature than the middle part of the press roller, much water evaporates from the lateral edges of the paper sheet. The paper sheet is inevitably curled.